3 Answers
3

I would recommend you have a look at this! it outlines everything that you would need to know! it should also be a very good starting point! the sandbox solutions can be found near the bottom what goes into detail. Yes it a big read but it answers all your questions

Run vstor40_x86.exe or vstor40_x64.exe to install the Visual Studio Tools for Office runtime. Choose the setup file that matches your
computer architecture (x86 or x64).

These setup files are available on the Visual Studio installation
media, and they are also available for download at
http://go.microsoft.com/fwlink/?LinkId=140384. The prerequisites for
the Visual Studio Tools for Office runtime are the same as the
prerequisites for the .NET Framework.

Install the language pack for the Visual Studio 2010 Tools for Office Runtime if you use non-English settings for Windows, and you
want to see runtime messages in the same language that you use for
Windows. If any end users run your solutions with non-English settings
for Windows, they must have the correct language pack to see runtime
messages in the same language as Windows. The language pack is
available at http://go.microsoft.com/fwlink/?LinkId=140386.

If you need ClickOnce non-English language support during the
deployment and installation of your solution, install the .NET
Framework Language Packs. The language packs are available from the
Microsoft Download Center.

Ok. Thanks:) and one more thing I wanted to ask is I have installed VS 2012 ultimate and I have office 365 preview. I just cant see the office developer tools for visual studio 2010..i cant even see ay templates for sharepoint/office either
–
sharmilaAug 23 '12 at 11:21

check out the edited answer for installing and configuring vs, the second link is documentation on hoto guides with office 365
–
ali SharepointAug 23 '12 at 11:42

I would not name web part as application and I never bother about sanboxing since it is impossible to have non-sandboxed apps in Office 365.

The easiest way for a newbie to develop a web part is to publish any Infopath form which then appear in list of available Infopath web parts. The latter you can see/use by navigating to or create in web browser to any web part ASPX page in Sharepoint online (Office 365) site > click Add a web part link > Forms (under Categories) > Infopath Form Web Part > Add > Click here to open the tool pane link > in tool pane under List or Library: list box you will see the list of published Infopath forms

The easiest way to create and publish an Infopath form is to navigate in browser to any list in Sharepoint online site > click Customize Form on ribbon > in Opened Infopath Designer 2010 click Quick Publish button (on ribbon File > Info) ... after or without its editing

That's it, you have developed, published and used ("sandboxed") web part.